Abstract: An experimental study is performed to investigate the e ects of phase difference on the mean thrust coe cient of two wings in tandem undergoing a two-dimensional (2-D) plunging motion in a low Reynolds number water tunnel. A 3-D force sensor and a 2-D digital particle image velocimetry (DPIV) are used to measure the wing thrust force and leading edge vortex (LEV) around the wings, respectively. The mean thrust coe cient of the forewing follows a sinusoidal curve when the phase difference changes from 0° to 360°.The increase of the mean thrust coe cient of the forewing is caused by the LEV and stagnation region of the hindwing enhancing the jet velocity behind the forewing and its e ective angle of attack. The curve of the mean thrust coe cient of the hindwing has a V-shaped feature as the phase difference increases. The decreased coe cient in the bottom of the V-shaped curve is caused by the vortex shed from the forewing restraining the LEV formation of the hindwing and reducing its e ective angle of attack. When the spacing distance is half chord and phase difference is 290°, the combined mean thrust coe cient of the forewing and hindwing can reach the maximum value of 0.667, over twice the reference single wing value (2×0.255).